## Contrasting Components of Diversity

It is often instructive to compare measures of diversity reflecting different components of diversity (richness and evenness) calculated from the same samples, as they may

Figure 3 Pictorial representation of Cairns' method of estimating Simpson's index by a process of sequential comparisons in two contrasting assemblages. Both assemblages have the same species richness (three species) but the first (a) is highly dominated, whereas the individuals in the second (b) are more evenly distributed among groups. Individuals are selected at random and each is compared with the previous individual. The number of times a dissimilar pair is encountered (cross) is counted to give the value (number of runs - 1), which is divided by the total number of individuals (N) to estimate 1 - A.

Figure 3 Pictorial representation of Cairns' method of estimating Simpson's index by a process of sequential comparisons in two contrasting assemblages. Both assemblages have the same species richness (three species) but the first (a) is highly dominated, whereas the individuals in the second (b) are more evenly distributed among groups. Individuals are selected at random and each is compared with the previous individual. The number of times a dissimilar pair is encountered (cross) is counted to give the value (number of runs - 1), which is divided by the total number of individuals (N) to estimate 1 - A.

show contrasting trends. As an example, in a standardized weak increasing trend in the numbers of species caught set of sample trawls from an area of the North Sea (Figure 2), but the Simpson evenness index shows a sampled annually between 1980 and 1990, there is a significant decreasing trend. Thus, the assemblage is clearly changing in terms of its dominance structure; although numbers of species may be increasing, the trend is for the assemblage to become less diverse. Interpreting a trend in one component of diversity without considering others may miss important changes in assemblage structure.

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